# GoGeometry Action 67!

- Author:
- Tim Brzezinski

- Topic:
- Congruence

Creation of this applet was inspired by a tweet from Antonio Gutierrez.
Below, you will find a circle inscribed in a square.
(You can move the vertices of the square around if you'd like.)

**Slide the slider slowly and carefully observe what happens.****How does this applet informally show that the area of the green triangle is 1/40 the area of the square?****How can we formally prove this true?**